TechBlog from Sewell

IDE Hard Drives are sometimes classified by their dimensions. For example, most desktop computers have 3.5 inch Hard Drives, older desktop computers have 5.25 inch drives. Laptops typically have 2.5 inch drives(some do have 1.8 inch drives) while ipods have 1.8 inch drives.

These dimensions are not completely accurate. 3.5 inch drives are actually about 4 inches wide. If you are trying to figure out what type of interface your drive has, the most accurate way is to count the number of pins or holes the drive has on the interface.

If the drive has 39-40 pins in one section, then your HD has a 3.5 inch interface. It is either a 3.5 inch or a 5.25 inch drive. If your HD has 43-44 pins (don’t count the 4 pins that are separated from the other 43) then it has the same interface as a 2.5 inch drive, and is either a 2.5 inch drive or a 1.8 inch drive. If your drive has 50 holes then it is most likely a 1.8 inch Toshiba drive(typical iPod drive).

To cap things off, there are three main interfaces:

40 pins= 3.5 and 5.25 inch drives.
44 pins=1.8 and 2.5 inch drives.
50 holes=1.8 inch Toshiba Drive.

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